Method and a device for measuring stress forces in refiners

ABSTRACT

Methods of measuring stress forces in refiners are disclosed in which the refiners include refining disks with a refining surface and refining bars extending across the refining surface, as well as a measuring surface comprising a portion of the refining surface, the measuring surface being movably mounted on the surface of at least one of the refining disks and a pair of rigidly mounted force sensors for producing oppositely directed deflections when the measuring surface is influenced by stress forces, the method comprising resiliently mounting the measuring surface in a direction parallel to the surface of the refining disk and calculating the stress force based on the difference between the deflections measured by the respective pairs of the force sensors. Apparatus measuring stress forces in such refiners are also disclosed.

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for measuringstress forces in refiners having refining disks that define a refininggap between them for refining material.

BACKGROUND OF THE INVENTION

Refiners such as those discussed above are used for refining fibrousmaterial. These refiners normally comprise refining members in the formof disks which rotate in relation to each other and between whichrefining material passes from the inner periphery of the refiningmembers, where the refining material is supplied, to the outer peripheryof the refining members, through a refining gap formed between therefining members. Often one of the refining disks is stationary whilethe other one rotates. The refining disks are generally constructed fromrefining segments provided with bars. The inner segments generally havea coarse pattern and the outer segments generally have a finer patternin order to achieve fine refining of the refining material.

To ensure good quality refining material when refining fibrous material,the disturbances in operating conditions that continually occur forvarious reasons are corrected by continuous control of the variousrefining parameters to optimal values. This can be achieved, forinstance, by altering the supply of water to give greater or lesscooling effect, by changing the flow of refining material, or byadjusting the distance between the refining members, or a combination ofthese measures. To enable the necessary adjustments and correctionscareful determination of the energy transmitted to the refining materialis necessary, as well as the distribution of the energy transmittedacross the surface of the refining members.

In order to determine the energy/power transmitted to the refiningmaterial it is known to attempt to measure the shearing forces occurringin the refining zone. It is known that a shearing force occurs when twosurfaces move in relation to each other with a viscous liquid betweenthem. Such a shearing force is also created in a refiner when refiningwood chips are mixed with water. It can be imagined that the wood chipsare both sheared and rolled between the refining disks, and also thatcollisions occur between wood chips and bars. The shearing force isdependent, inter alia, on the force of the disks as they are broughttogether as well as the friction coefficient. Furthermore, the normalforce exerted on the surface varies with the radius.

A method and a measuring device are known for measuring stress forces insuch refiners, as shown in International Application No. WO 00/78458,comprising a force sensor that measures the stress forces over ameasuring surface constituting a part of a refining disk, and where themeasuring surface comprises at least parts of more than one bar and isresiliently arranged in the surface of the refining disk. However, thismeasuring device has proved to be very sensitive to temperaturevariations, which are common in the relevant conditions, and ittherefore often gives incorrect values for the stress, which cannot beused to control the refining process.

One object of the present invention is to solve the problems mentionedabove and to thus provide a method and a measuring device that providesa more reliable result than known devices.

SUMMARY OF THE INVENTION

In accordance with the present invention, this and other objects havenow been realized by the discovery a method of measuring stress forcesin refiners including a pair of relatively rotatably refining disksjuxtaposed with a refining gap therebetween for refining material withinthe refining gap, each of the pair of refining disks including arefining surface and a plurality of refining bars extending across therefining surface, and a measuring surface comprising a predeterminedportion of the refining surface of at least one of the refining disksincluding at least a portion of a plurality of the refining bars, themeasuring surface being movably mounted on the surface of the at leastone refining disk, and a pair of rigidly mounted force sensors forproducing oppositely directed deflections when the measuring surface isinfluenced by stress forces, the method comprising resiliently mountingthe measuring surface in a direction parallel to the surface of the atleast one refining disk and calculating the stress force based on thedifference between the deflections measured by the respective pairs ofthe force sensors. Preferably, the method includes calculating themagnitude and distribution of power transmitted to the refining materialbased on the difference between the deflections measured by therespective pairs of the force sensors and controlling the refiningprocess using the calculated values.

In accordance with the present invention, this and other objects havenow also been realized by the invention of apparatus for measuringstress forces in a refiner including a pair of relatively rotatablerefining disks juxtaposed with a refining gap therebetween for refiningmaterial within the refining gap, each of the pair of refining disksincluding a refining surface including a plurality of refining barsextending across the refining surface, the apparatus comprising ameasuring surface comprising a predetermined portion of the surface ofat least one of the refining disks and including at least a portion of aplurality of the refining bars, the measuring surface being resilientlymounted on the surface of the at least one of the refining disks, a pairof force sensors producing oppositely directed deflections when themeasuring surface is influenced by the stress forces, and a bodyconnecting the pair of force sensors to the measuring surface, wherebythe stress forces can be calculated based on the difference between thedeflections measured for the pair of forced sensors. Preferably, themeasuring surface includes a central axis substantially perpendicular tothe measuring surface and wherein the pair of force sensors aresymmetrically disposed with respect to the central axis.

In accordance with one embodiment of the apparatus of the presentinvention, each of the pair of force sensors abuts the body, and theapparatus includes attachment means for affixing the pair of forcesensors with respect to the body. In a preferred embodiment, the bodyincludes an extending portion disposed distal from the measuringsurface, the extending portion of the body including a joint whereby thebody is pivotable about the joint in a direction substantially parallelto the surface of the at least one of the refining disks.

In accordance with another embodiment of the apparatus of the presentinvention, the apparatus includes a seal member surrounding themeasuring surface, the seal member comprising a yieldable material. In apreferred embodiment, the body includes a first end connected to themeasuring surface and a second opposite end, the apparatus furthercomprising a housing containing the pair of force sensors and the body,and attachment means for attaching the pair of force sensors to thehousing, the second opposite end of the body affixed in the housing, andthe measuring surface and the seal member sealing the housing.Preferably, the apparatus includes a sleeve, with the sealing memberbeing disposed in the sleeve, whereby the sealing member and themeasuring surface close the housing.

In accordance with the method of the present invention, the measurementtakes place by the measuring surface being resiliently mounted in adirection parallel to the surface of the refining disk and, in the eventof a stress force, being movable in that direction in relation to tworigidly mounted force sensors with which the measuring surface isconnected and which are arranged to produce oppositely directeddeflection when the measuring surface is influenced by those stressforces, and the stress forces are calculated on the basis of thedifference between the deflections measured for respective force sensorson each occasion. Using two force sensors offers the important advantagethat a value can be obtained for the stress forces that is not affectedby any temperature variations which occur. This is done by utilizing thedifference between the deflections measured for respective force sensorson each occasion as a value of the stress forces. This value can then beused to calculate the magnitude and distribution of the powertransmitted to the refining material and these calculations can then beutilized to control the refining process.

The use of two sensors, in the manner described in accordance with thepresent invention, renders the advantage that any error in measurementis halved.

A preferred embodiment of the measuring device in accordance with thepresent invention is one in which the device comprises members thatmeasure the stress forces in the form of two force sensors arranged toproduce oppositely directed deflection when the measuring surface isinfluenced by such stress forces, so that the stress forces can becalculated on the basis of the difference between the deflectionsmeasured for respective force sensors on each occasion, and it alsocomprises a body connecting the force sensors to the measuring surface.The advantages of using two force sensors have been described above andare of great significance in this context.

In accordance with an advantageous embodiment of the present inventionthe force sensors are arranged symmetrically, i.e. symmetrically inrelation to a central axis of the measuring surface that isperpendicular to the measuring surface.

The sensors, which are preferably piezoelectric force sensors (alsoknown as transducers), constructed out of quartz crystal (so-calledquartz sensors), also contribute to an extremely rigid measuring devicebeing possible. The preferred sensors can handle up to 200° C. and arealso linear up to this temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described with reference to thefollowing detailed description which refers to the accompanyingschematic drawings, in which

FIG. 1 is a top perspective view of a refining segment in a refiningdisk provided with measuring devices in accordance with the presentinvention,

FIG. 2 is a side elevational, diagrammatic representation of a measuringdevice in accordance with the present invention,

FIG. 3 a is a diagrammatic representation of the force ratios applicableto the present invention,

FIG. 3 b is another diagrammatic representation of the force ratiosapplicable to the present invention; and

FIG. 4 is a side, elevational, sectional view of a measuring device inaccordance with the present invention.

DETAILED DESCRIPTION

FIG. 1 shows a part of a refining disks in the form of a refiningsegment 1 provided with a pattern comprising a number of bars 3extending primarily in the radial direction. In this figure measuringdevices 5 in accordance with the present invention have beenschematically indicated. These measuring devices preferably have acircular measuring surface with a diameter in the order of 30 mm, forinstance, but the measuring surface may also have some other geometricshape.

The measuring devices are preferably arranged at different radialdistances from the center of the refining disk, and segments atdifferent distances from the center preferably also have measuringdevices. The measuring devices may also advantageously be peripherallydisplaced in relation to each other, these measures being aimed at beingable to better determine the power distribution in the refiner and thusto better control the refining process. When a measuring device isinfluenced by a force parallel to the surface of the refiningdisk/segment, each force sensor of the measuring device will generate asignal that is proportional to the load.

The measuring device in accordance with the present invention functionsin accordance with the principle illustrated in FIG. 2. This shows adisk segment 1 from the side, provided with bars 3. A measuring device 5is also shown which, for the sake of simplicity, is shown as comprisingonly one force sensor 10, and a measuring surface 7 in the form of aportion of the surface of the disk segment, which is provided with anumber of bars 6, or at least parts thereof. When the refining disk issubjected to a shearing load F the measuring device 5 (the sensor) willtake up a load F_(m) which is represented by the following expression:$\begin{matrix}{F_{m} = {F \cdot \frac{1_{1}}{1_{2}}}} & (1)\end{matrix}$where l₂ is the distance between the location where the sensor 10 isattached in the measuring device and a joint 8 in the device, and wherel₁ is the distance between the measuring surface 7 of the measuringdevice and the joint 8. This formula is valid provided the joint 8 doesnot take up any torque and that the pressure distribution over themeasuring surface 7 subjected to the shearing force is not too uneven.In principle the joint 8 consists of a plate that is so thin that itcontributes negligibly to the total rigidity of the measuring devicewhile at the same time being able to withstand the loads it is subjectedto. The thickness of the plate may be relatively great since therigidity of the sensor is relatively great, thus resulting in onlyslight deflection of the plate. The dimensions of the joint 8 shall thusbe suitable for withstanding the vertical load arising while at the sametime absorbing only a negligible part of the lateral load that the screwand the sensor shall absorb. See also the detailed description withreference to FIG. 4.

The models in FIGS. 3 a and 3 b depict how high or low rigidity affectsthe function of the measuring device through the rigidity of the sensor,the attachment screw (the attachment member by which each sensor issecured in relation to the measuring surface and the body, see FIG. 4)and joint. The force and the torque absorbed by the sensor/attachmentscrew and joint, respectively, are controlled by the equationF_(sensor)=k₂·δ and M=k₃·Δφ, where M is the torque in the joint. k₂ isthe rigidity of the spring 15, i.e. the sensor 10 together with theattachment screw 20, and the rigidity k₃ is the rigidity of the supportpoint/joint 8. The equation shows clearly that if F is constant and k₂increases then δ will decrease, as will also M since the torque isdirectly proportional to the deflection δ for small angles. In thepresent case k₂ is large, which means that equation (1) is valid.

It should be emphasized that by relatively high rigidity of thesensor/attachment screw is meant in the present case high rigidity inrelation to the load the sensor/screw shall absorb. The load may varyconsiderably over the refining zone—from some 20N to some 150N, forinstance. With an estimated average value of about 40N displacements ofthe measuring surface obtained in the present case can be measured inthe order of hundredths of a millimeter. As mentioned earlier, thesesmall displacements facilitate sealing of the device from thesurrounding environment, for instance. As regards the body 17, this canbe deemed completely rigid in a direction perpendicular to the measuringsurface.

FIG. 4 shows a preferred embodiment of a measuring device in accordancewith the present invention. The measuring device 5 comprises a measuringsurface 7 provided with bars 6, or parts of bars, which measuringsurface constitutes a part of a disk segment, as illustrated in FIG. 1.As can also be seen in FIG. 1, the measuring device preferably has acircular measuring surface.

The measuring surface 7 is in direct contact with a body 17, preferablyof steel, which extends through the interior of the device. Themeasuring surface is preferably firmly screwed in the body 17. A shortdistance below the measuring surface the body 17 is provided with atransverse recess in which two force sensors, 10 and 11, are arranged,preferably quartz sensors. The sensors, 10 and 11, are fixed in relationto the body 17 by means of attachment screws 20 arranged to clamp eachsensor against the body 17 on diametrically opposite sides thereof, aswill be further described below. The attachment screws and anyintermediate elements are preferably shaped so that a uniformlydistributed load is obtained on each sensor, and preferably with acertain pre-stress. In accordance with this embodiment the sensors arearranged symmetrically in relation to a center line extending throughthe measuring surface 7 and the body 17. The sensors will thus produceoppositely directed deflection when influenced by a force. When thepressure on the measuring surface increases, therefore, the load willincrease on one of the sensors and will simultaneously decrease on theother. Naturally it would be possible to arrange the sensors in someother way in relation to each other and still have their deflectionoppositely directed. Other attachment devices for the sensors, 10 and11, are naturally also possible.

The body 17 preferably has a circular cross section. Further down, belowthe sensors the body 17 assumes a narrowing, flattened shape within asurface corresponding to the joint 8, mentioned previously and describedwith reference to FIGS. 2, 3 a and 3 b.

The load F_(m) which the measuring device will take up through thesensors, 10 and 11, when it is subjected to a shearing force F iscalculated in this case as: $\begin{matrix}{F_{m} = {\frac{S_{2} - S_{1}}{2} \cdot k}} & (2)\end{matrix}$where S₁ is the shearing force indicated by the first sensor 10, S₂ isthe shearing force indicated by the second sensor 11 and k is a scalefactor based on previous calibrations.

This means that the shearing load F influencing the refining disk can becalculated as: $\begin{matrix}{F = {\frac{1_{2}}{1_{1}} \cdot \frac{S_{2} - S_{1}}{2} \cdot k}} & (3)\end{matrix}$

This is the equation used to calculate the magnitude and distribution ofthe power transmitted to the refining material, these calculations thenbeing utilized to control the refining process.

The sensors, 10 and 11, and the body 17 are arranged in a protectivehousing 22. This housing has an opening at the top abutting thesurrounding refining segment, which is closed by the measuring surface7, a seal 12 surrounding the measuring surface, and by a sleeve 13 inwhich the seal is arranged. The seal 12 consists of a particularlysuitable, somewhat yielding material such as rubber, so that it canpermit the small movements caused by the shearing forces in themeasuring surface while still achieving a good seal that prevents steamand pulp from penetrating into the device. The seal preferably also hasa damping effect on the vibrations that arise during operation. Thepurpose of the sleeve 13 is primarily to facilitate closing of themeasuring device since the measuring surface and the seal are firstmounted in the sleeve which can then easily be partially inserted intothe housing 22. It is possible to omit the sleeve.

The housing 22 also has a function when it comes to fixing the sensors,10 and 11, in relation to the measuring surface 7. The sensors are thusattached in the housing by means of attachment screws 20. Finally, thebody 17 is attached in the housing at the end opposite to the measuringsurface. Although the invention herein has been described with referenceto particular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

1. A method of measuring stress forces in refiners including a pair ofrelatively rotatable refining disks juxtaposed with a refining gaptherebetween for refining material within said refining gap, each ofsaid pair of refining disks including a refining surface and a pluralityof refining bars extending across said refining surface, and a measuringsurface comprising a predetermined portion of said refining surface ofat least one of said refining disks including at least a portion of aplurality of said refining bars, said measuring surface being movablymounted on said surface of said at least one refining disk, and a pairof rigidly mounted force sensors for producing oppositely directeddeflections when said measuring surface is influenced by stress forces,said method comprising resiliently mounting said measuring surface in adirection parallel to said surface of said at least one refining diskand calculating said stress force based on the difference between thedeflections measured by said respective pairs of said force sensors. 2.The method of claim 1 including calculating the magnitude anddistribution of power transmitted to said refining material based on thedifference between the deflections measured by said respective pairs ofsaid force sensors and controlling said refining process using saidcalculated values.
 3. Apparatus for measuring stress forces in a refinerincluding a pair of relatively rotatable refining disks juxtaposed witha refining gap therebetween for refining material within said refininggap, each of said pair of refining disks including a refining surfaceincluding a plurality of refining bars extending across said refiningsurface, said apparatus comprising a measuring surface comprising apredetermined portion of said surface of at least one of said refiningdisks and including at least a portion of a plurality of said refiningbars, said measuring surface being resiliently mounted on said surfaceof said at least one of said refining disks, a pair of force sensorsproducing oppositely directed deflections when said measuring surface isinfluenced by said stress forces, and a body connecting said pair offorce sensors to said measuring surface, whereby said stress forces canbe calculated based on the difference between said deflections measuredfor said pair of force sensors.
 4. The apparatus of claim 3 wherein saidmeasuring surface includes a central axis substantially perpendicular tosaid measuring surface and wherein said pair of force sensors aresymmetrically disposed with respect to said central axis.
 5. Theapparatus of claim 3 wherein each of said pair of force sensors abutssaid body, and including attachment means for affixing said pair offorce sensors with respect to said body.
 6. The apparatus of claim 5wherein said body includes an extending portion disposed distal fromsaid measuring surface, said extending portion of said body including ajoint whereby said body is pivotable about said joint in a directionsubstantially parallel to said surface of said at least one of saidrefining disks.
 7. The apparatus of claim 3 including a seal membersurrounding said measuring surface, said seal member comprising ayieldable material.
 8. The apparatus of claim 7 wherein said bodyincludes a first end connected to said measuring surface and a secondopposite end, said apparatus further comprising a housing containingsaid pair of force sensors and said body, and attachment means forattaching said pair of force sensors to said housing, said secondopposite end of said body affixed in said housing, and said measuringsurface and said seal member sealing said housing.
 9. The apparatus ofclaim 8 including a sleeve, with said sealing member being disposed insaid sleeve, whereby said sealing member and said measuring surfacecloses said housing.